Fuel cells and fuel cell systems containing non-aqueous electrolytes

ABSTRACT

Fuel cells and fuel cell systems that include at least one non-aqueous electrolyte. Fuel cells according to the present disclosure may include an anode region that is adapted to receive an anode feed stream containing chemically bound hydrogen and which liberates protons at an anode of the fuel cell, a cathode region that is adapted to receive a stream containing oxygen, and an electrolytic barrier that separates the anode and cathode regions and which contains a non-aqueous electrolyte. The non-aqueous electrolyte is preferably acidic or basic. The electrolyte may have an acid ionization constant (K a ) greater than 5×10 −6  at 25° C. if the non-aqueous electrolyte is an acid and a base ionization constant (K b ) greater than 5×10 −6  at 25° C. if the non-aqueous electrolyte is a base. The fuel cell has an operating temperature of less than 300° C., and may operate at temperatures above, at, and below 100° C.

RELATED APPLICATIONS

[0001] This application is a continuation of and claims priority to U.S.patent application Ser. No. 09/872,743, now U.S. Pat. No. 6,667,128,which was filed on Jun. 1, 2001, and which claims priority to U.S.Provisional Patent Application Serial No. 60/208,880, which was filed onJun. 1, 2000. The complete disclosures of the above-identifiedapplications are hereby incorporated by reference for all purposes.

FIELD OF THE DISCLOSURE

[0002] The present invention relates generally to fuel cells and fuelcell systems, and more particularly to fuel cells and fuel cell systemsthat contain a non-aqueous electrolyte.

BACKGROUND OF THE DISCLOSURE

[0003] An electrochemical fuel cell is a device that reacts a fuelsource with an oxidizing agent to produce an electric current. Commonly,the fuel source is hydrogen gas, and the oxidizing agent is oxygen. Anexample of a fuel cell utilizing these reactants is a proton exchangemembrane fuel cell (PEMFC or PEM fuel cell), in which hydrogen gas iscatalytically dissociated in the fuel cell's anode chamber into protonsand electrons. The liberated protons are drawn through an electrolyticmembrane into the fuel cell's cathode chamber. The electrons cannot passthrough the membrane and instead must travel through an external circuitto reach the cathode chamber. In the cathode chamber, the protons andelectrons react with oxygen to form water. The net flow of electronsfrom the anode to the cathode chamber produces an electric current,which can be used to meet the electrical load being applied to the fuelcell by an associated electrical, or energy-consuming, device, such as avehicle, boat, light, appliance, household, etc.

[0004] The fuel cell's ability to transport hydrogen ions across theelectrolytic membrane is a function of the hydration of the membrane. Inthe case of low-temperature fuel cells, such as PEM fuel cells andalkaline fuel cells (AFCs), the ionically-conductive electrolyte is awater-swollen, strongly acidic polymeric membrane (PEMFC) or an aqueoussolution of a strong base such as potassium hydroxide (AFC). Theseionically-conductive electrolytes are susceptible to drying (losingwater) or flooding (absorbing excess water). Either occurrence can leadto poor performance of the fuel cell and premature failure. U.S. Pat.No. 6,059,943, which is incorporated herein by reference in its entiretyfor all purposes, describes many of the problems related to maintaininga correct water balance in electrochemical devices such as fuel cells.

SUMMARY OF THE DISCLOSURE

[0005] The present disclosure includes fuel cells and fuel cell systemsthat include a non-aqueous electrolyte. Fuel cells according to thepresent disclosure include an anode region that is adapted to receive ananode feed stream containing chemically bound hydrogen and whichliberates protons at an anode of the fuel cell, a cathode region that isadapted to receive a stream containing oxygen, and an electrolyticbarrier that separates the anode region from the cathode region andwhich contains a non-aqueous electrolyte. The non-aqueous electrolyte ispreferably acidic or basic. The electrolyte may have an acid ionizationconstant (K_(a)) greater than 5×10⁻⁶ at 25° C. if the non-aqueouselectrolyte is an acid and a base ionization constant (K_(b)) greaterthan 5×10⁻⁶ at 25° C. if the non-aqueous electrolyte is a base. The fuelcell has an operating temperature of less than 300° C., and may operateat temperatures above, at, and below 100° C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a schematic diagram of a fuel cell system constructedaccording to the present invention.

[0007]FIG. 2 is a schematic diagram of another fuel cell systemconstructed according to the present invention.

[0008]FIG. 3 is a schematic diagram of a fuel cell constructed accordingto the present invention.

[0009]FIG. 4 is a schematic diagram of a fuel processor suitable for usein the fuel cell system shown in FIG. 2.

[0010]FIG. 5 is a schematic diagram of another fuel processor suitablefor use in the fuel cell system shown in FIG. 2.

DETAILED DESCRIPTION AND BEST MODE OF THE DISCLOSURE

[0011] A fuel cell system according to the present disclosure is shownin FIG. 1 and generally indicated at 10. System 10 includes a fuel cellstack 12 and a source 14 of an anode feedstock 16, which is delivered tofuel cell stack 12 as stream 17. Anode feedstock 16 is any suitablecomposition or compositions that contain chemically bound hydrogen andwhich liberate protons at the anode of a fuel cell. An example of asuitable anode feedstock 16 is hydrogen gas. Other examples includemethanol, hydrazine, ethanol and formaldehyde. Source 14 may include astorage tank or other reservoir containing feedstock 16, which isdelivered to fuel cell stack 12 through any suitable mechanism, such asby pumping or by gravity flow. For example, source 14 may be a tank ofcompressed hydrogen gas, a tank or other fluid-holding container ofmethanol, etc. Another example is a hydride bed, which contains storedhydrogen gas.

[0012] Alternatively, source 14 may include one or more devices that areadapted to produce anode feedstock 16, such as by a chemical reaction.An example of such a source 14 is shown in FIG. 2 and includes at leastone fuel processor 18. Fuel processor 18 is adapted to receive a feedstream 20 that contains the feedstock for the fuel processor and isdelivered to the fuel processor by any suitable mechanism, such as bypumping or by gravity flow. Fuel processor 18 produces an anodefeedstock 16 in the form of hydrogen gas 22 from feed stream 20.Preferably, the fuel processor is adapted to produce substantially purehydrogen gas, and even more preferably, the fuel processor is adapted toproduce pure hydrogen gas. For the purposes of the present disclosure,substantially pure hydrogen gas is greater than 90% pure, preferablygreater than 95% pure, more preferably greater than 99% pure, and evenmore preferably greater than 99.5% pure. Suitable fuel processors forproducing hydrogen gas are disclosed in U.S. Pat. Nos. 6,221,117,5,997,594 and 5,861,137, and U.S. Provisional Patent Application SerialNo. 60/188,993, which was filed on Mar. 13, 2000 and is entitled “FuelProcessor,” each of which is incorporated by reference in its entiretyfor all purposes.

[0013] Non-exclusive examples of suitable mechanisms by which fuelprocessor 18 produces hydrogen gas 22 include steam reforming,autothermal reforming, pyrolysis, partial oxidation, electrolysis anddissociation by irradiation. The composition and number of individualstreams forming feed stream 20 will tend to vary depending on themechanism by which fuel processor 18 is adapted to produce hydrogen gas22. For example, if fuel processor 18 produces hydrogen gas by steam orautothermal reforming, feed stream 20 contains a carbon-containingfeedstock 24 and water 26, such as shown in FIG. 2. If fuel processor 18produces hydrogen gas by pyrolysis, irraditation or catalytic partialoxidation of a carbon-containing feedstock, feed stream 20 contains acarbon-containing feedstock and does not include water. If fuelprocessor 18 produces hydrogen gas by electrolysis, feed stream 20contains water and does not contain a carbon-containing feedstock. Whenthe feed stream contains water and a carbon-containing feedstock that issoluble with water, the feed stream may be a single stream, such asshown in a solid line in FIG. 2. When the carbon-containing feedstock isnot miscible in water, the water and carbon-containing feedstock aredelivered in separate feed streams 20, such as shown in dashed lines inFIG. 2.

[0014] Examples of carbon-containing feedstocks 24 include alcohols andhydrocarbons. Illustrative examples of suitable hydrocarbons includemethane, propane, natural gas, diesel, kerosene, gasoline and the like.Illustrative examples of suitable alcohols include methanol, ethanol,propanol, and polyols, such as ethylene glycol and propylene glycol. Apreferred, but by no means exclusive, feedstock for a fuel processor inthe form of a steam reformer is methanol, and more particularly,fuel-cell grade methanol. Generally, catalysts that are used forconducting the water-gas shift reaction are suitable for steam reformingmethanol. Commonly used (and commercially available) methanolsteam-reforming catalysts consist of mixtures of copper and zinc oxide,and copper and chromium oxide. These catalyst formulations are veryrapidly and completely poisoned by compounds of sulfur, compounds ofphosphorous, volatile heavy metals (e.g., cadmium, mercury), andcompounds of heavy metals. Therefore, the methanol is preferably freefrom these compounds so that the reforming catalyst is not poisoned.Similarly, other carbon-containing feedstocks should be sufficientlyfree from these or other compounds that will poison the reforming orother catalysts used to produce hydrogen gas therefrom.

[0015] Fuel cell stack 12 is adapted to produce an electric current 28from the portion of anode feedstock 16 (or hydrogen gas 22) deliveredthereto. Illustrative examples of suitable conventional fuel cellsinclude proton exchange membrane (PEM) fuel cells and alkaline fuelcells. Fuel cell stack 12 typically includes a plurality of fuel cells30 integrated together between common end plates 32, which contain fluiddelivery/removal conduits (not shown). The number of cells in stack 12may vary, such as depending upon such factors as the desired poweroutput, the size limitations of the system, and the maximum availablehydrogen (or other anode feedstock) supply. However, it is within thescope of the present disclosure that fuel cell stack 12 may include asingle fuel cell 30, or multiple fuel cells 30. Typically, fuel cellstack 12 receives all or at least a substantial portion of stream 17 andproduces electric current 28 therefrom. This current can be used toprovide electrical power to an associated energy-consuming device 34,such as a vehicle or a house or other residential or commercialdwelling.

[0016] Illustrative examples of devices 34 include, but should not belimited to, a motor vehicle, recreational vehicle, boat, tools, lightsor lighting assemblies, appliances (such as household or otherappliances), household, signaling or communication equipment, etc. Itshould be understood that device 34 is schematically illustrated inFIGS. 1 and 2 and is meant to represent one or more devices orcollection of devices that are adapted to draw electric current from thefuel cell system. By “associated,” it is meant that device 34 is adaptedto receive electrical power generated by stack 12. It is within thescope of the disclosure that this power may be stored, modulated orotherwise treated prior to delivery to device 34. Similarly, device 34may be integrated with stack 12, or simply configured to draw electriccurrent produced by stack 12, such as via electrical power transmissionlines.

[0017] In FIG. 3, an illustrative example of a fuel cell 30 constructedaccording to the present disclosure is shown. Fuel cell 30 includes ananode region 36 and a cathode region 38. The regions respectivelyinclude anode and cathode electrodes 40 and 42, which are schematicallyillustrated in FIG. 3. Any suitable electrode construction may be used.As an illustrative example, the electrodes may be porous and contain anelectrocatalyst 44 and an electrically conductive support 46 that is inelectrical communication with a current collection device 48 from whichenergy-consuming device 34 may directly or indirectly draw electriccurrent 28.

[0018] The anode region 36 of the fuel cell receives at least a portionof stream 17. Anode region 36 is periodically purged, and releases apurge stream 50, which may contain hydrogen gas. Alternatively, hydrogengas may be continuously vented from the anode region of the fuel cell.The purge streams may be vented to the atmosphere, combusted, used forheating, fuel or as a feedstock for the fuel processing assembly. Thefuel cells of a fuel cell stack may exhaust a common purge streamconsisting of the purge streams from the individual fuel cells containedtherein. The purge streams from the fuel cells may be integrated into asuitable collection assembly through which the combined purge stream maybe used for fuel, feedstock, heating, or otherwise harvested, utilizedor stored.

[0019] Cathode region 38 receives a stream 52 containing oxygen 54, suchas a stream of pure oxygen, an air stream, or an air stream that hasbeen enriched with oxygen gas, and releases a cathode air exhaust stream56 that is partially or substantially depleted in oxygen. The relativeflow rate of air will generally be greater than that of pure oxygenbecause of the lower relative concentration of oxygen atoms provided.

[0020] The anode and cathode regions are separated by an electrolyticbarrier 60 through which hydrogen ions (protons) may pass. Electronsliberated from hydrogen gas 22 (or other composition or compositionsthat contain chemically bound hydrogen and which liberate protons at theanode of the fuel cell stack) cannot pass through barrier 60, andinstead must pass through an external circuit 61, thereby producingelectric current 28 that may be used to meet the load applied by device34. Current 28 may also be used to power the operation of the fuel cellsystem. The power requirements of the fuel cell system are collectivelyreferred to as the balance of plant requirements of the fuel cellsystem.

[0021] Electrolytic barrier 60 includes a non-aqueous electrolyte 62 anda support structure, or containment structure, 64. Illustrative examplesof suitable support structures 64 include absorbent media that absorbelectrolyte 62 and porous structures. Illustrative examples of absorbentmedia include swollen polymer mats and films. Examples of suitablepolymers are sold by DuPont under the trade name NAFION™, which isavailable in varying thicknesses, such as between 0.001 inches to 0.011inches. Generally thinner sheets are preferred because they decrease thediffusion distance across the electrolyte. Illustrative examples ofporous structures include mats, meshes, or channels, which contain theelectrolyte by capillary action and/or the surface tension between theelectrolytes and the porous structures. The support structure may besaturated with the electrolyte. Alternatively, less than full saturationmay be used without adversely affecting the conductivity of theelectrolyte, especially when the electrolyte is hydrophobic.

[0022] Electrolyte 62 may have a low or even exceptionally low vaporpressure, low viscosity, a melting point that is less than 100° C. andpreferably less than 25° C. The electrolyte may have a high ionicconductivity and a range of electrochemical potential of at least 0.6 Vand preferably at least 1.5 V, over which components of the electrolyte(other than protons or hydroxide ions) are neither oxidized nor reducedat the electrodes.

[0023] In conventional low-temperature fuel cells with water-basedelectrolytes, water is volatile at the operating temperature of the fuelcell. As used herein, the term “low-temperature” is used to refer totemperatures less than 100° C., “high-temperature” is used to refer totemperatures greater than 300° C., with “moderate-temperature” referringto temperatures between 100° C. and 300° C. Because water is volatile atthe operating temperature of low-temperature fuel cells, the amount ofwater in the electrolyte must be continuously monitored to ensure theelectrolyte is not dried from too little water or flooded from too muchwater. Too little water decreases the ionic conductivity of theelectrolyte, while too much water results in droplets that impede theconduction of ions across the electrolyte. Furthermore, in fuel cells inwhich aqueous electrolytes are used, the fuel cell is typicallyconfigured to remove formed water so that water does not significantlyincrease the volume of the electrolyte. This water monitoring andmaintenance requires significant monitoring and/or control to maintainthe fuel cell's operability and efficiency.

[0024] A method for managing and removing excess water is to operate thefuel cell, or fuel stack, at an operating temperature at or above 100°C. Operating the fuel cell at this temperature results in water beingvaporized. As a result, the water cannot significantly add to the volumeof the electrolyte. However, low-temperature water-based electrolytescannot be used above 100° C., and high-temperature inorganicelectrolytes do not have sufficient ionic conductivity to function atlow and/or moderate temperatures. In fact, some high-temperatureelectrolytes are not even liquids at low or even moderate temperatures.Examples of fuel cells that operate at temperatures greater than 100° C.include phosphoric acid fuel cells (180-200° C.), molten carbonate fuelcells (450-550° C.) and solid oxide fuel cells (600-700° C.).

[0025] Electrolyte 62 may exhibit very low solubility for water by usinghydrophobic anions and cations. Because non-aqueous electrolyte 62 isinsoluble or only slightly soluble in water, it produces a fuel cell 30in which the amount of water in the electrolytic barrier 60 does notneed to be managed.

[0026] Non-aqueous electrolyte 62 enables fuel cell 30 to be operated atmoderate temperatures as well as at the low-temperature conditions atwhich aqueous electrolytes are typically operated. Worded another way,the ionic conductivity of fuel cells 30 is acceptable for operation ofthe fuel cells at both low and moderate temperatures. Therefore, it iswithin the scope of the disclosure that fuel cell 30 (and stack 12) willbe operated at a range of temperatures in the range of 0° C. (orapproximately 0° C.) and 300° C. (or approximately 300° C.). In manyembodiments, fuel cell 30 will be operated at temperatures in the rangeof approximately 15° C. and approximately 200° C., and more typically inthe range of approximately 15° C. and approximately 150° C. It is alsowithin the scope of the present disclosure that the fuel cell, or fuelcell stack, may be operated at temperatures in the range of 0° C. or 15°C. and 100° C., at 100° C., and at temperatures greater than 100° C.,such as temperatures up to 200° C. or 300° C.

[0027] An advantage of operating at an elevated temperature is that theefficiency of the fuel cell increases with increases in its operatingtemperature. As the temperature increases, the electrocatalysts on theanode and cathode sides become more active. Also, the anode catalystbecomes less sensitive to gas-phase impurities. Another result ofoperating the fuel cell at a higher operating temperature is that theexhaust streams will be at a higher temperature than correspondingstreams of a low-temperature fuel cell. Accordingly, these streams willhave an increased heating value, which may be harvested through heatexchange or which may remove or lessen the need to heat these streamsprior to downstream use. Similarly, feed streams 17 and 52 may bedelivered at a higher temperature than conventionally used withlow-temperature fuel cells, thereby reducing or eliminating the need tocool these streams if they are available at temperatures higher than thetemperatures acceptable for use in low-temperature fuel cells.

[0028] Non-aqueous electrolyte 62 may be either a pure compound or amixture of compounds in which water is not a major component. Typically,the non-aqueous electrolyte should be less than 5% water on a molarbasis. Because the electrolyte serves as a barrier to electrons, but notprotons, the non-aqueous electrolyte should be a poor electricalconductor and a good ionic conductor. The non-aqueous electrolyte shouldhave low volatility, meaning that the normal boiling point of theelectrolyte is at least 90° C. and preferably at least 130° C., and morepreferably at least 150° C. The non-aqueous electrolyte should have amelting point of less than 5° C. and more preferably less than 0° C.

[0029] Examples of non-aqueous electrolytes 62 are inorganic acids suchas phosphoric acid or sulfuric acid, and low volatility hydrocarbonssuch as decane or hexadecane. Alternatively, the non-aqueous electrolytemay be a mixture of several compounds. These mixtures may include ahydrocarbon solvent combined with an acidic or basic solute, apolyalcohol solvent combined with an acidic or basic solute, a polyethersolvent combined with an acidic or basic solute, an organic ionic liquidsolvent combined with an acidic or basic solute, or any organic solventwith a normal boiling point of greater than 90° C. combined with anacidic or basic solute. The acidic solute should be a solute that isionized in solution to yield protons. The basic solute should be asolute that is ionized in solution to yield hydroxide ions. Examples ofsuitable solvents are hexadecane, decane, kerosene, propylene carbonate,propylene glycol, o-dichlorobenzene, and 1,3,5-trichlorobenzene.Examples of suitable acidic solutes are hydrogen hexafluorphosphate,hydrogen tetraphenylborate, sulfuric acid, and perchloric acid. Examplesof suitable basic solutes are tetramethylammonium hydroxide,tetraethylammonium hydroxide, tetrapropylammonium hydroxide,tetrabutylammonium hydroxide and other tetraalkylammonium hydroxides.

[0030] Another example of a non-aqueous electrolyte is an organic ionicliquid. Organic ionic liquids are discussed, for example, in M.Freemantle, “Eyes on Ionic Liquids,” Chemical Engineering and News, May15, 2000, p. 37-50, which is hereby incorporated by reference in itsentirety for all purposes. In general an organic ionic liquid includesan organic salt that is liquid at room temperature, (i.e. the meltingpoint of the salt is lower than room temperature) and ionization occurswhen the salt is molten or dissolved.

[0031] The chemistry behind an organic ionic liquid forming theelectrolyte in a fuel cell is illustrated below. Consider, for example,1-ethyl-3-methyl-imidazoleum tetrafluoroborate, which is an organicionic liquid (melting point 15° C. and boiling point >350° C.) that iscommercially available from Aldrich Chemical Co. in Milwaukee, Wis. Forthis compound to serve as the electrolyte in a PEM fuel cell, theconjugate base would be the tetrafluoborate ion (BF₄ ⁻), which isprotonated at the anode to form the conjugate acid HBF₄. Other anionscan be selected rather than tetrafluoborate for use with the1-ethyl-3-methyl-imidazoleum cation. For example, other suitable anionsinclude tetraphenylborate (BPh₄ ⁻), and hexafluorophosphate (PF₆ ⁻).

[0032] Examples of organic ionic liquids include 1,3-dialkylimidazoleumcations coupled to hydrophobic anions or hydrophylic anions. Specificexamples include 1-buty-3-methylimidazolium hexafluorophosphate,1-butylpyridinium nitrate, 1-ethyl-3-methylimidazoleumbis(trifluoromethanesulfonate), and 1-ethyl-3-methylimidazoleumbis(trifluoromethanesulfonyl)imide. The organic ionic liquid or liquidsmay themselves form electrolyte 62 or they may be the solvent for anacid or base, such as these discussed herein.

[0033] A preferred non-aqueous electrolyte 62 is either a strong acid ora strong base, since the electrolyte must serve as either a proton donoror a hydroxide-ion donor. For the purposes of the present disclosure, astrong acid is defined as an acid having an ionization constant (K_(a)),or first ionization constant if it is a polyproptic acid, greater than5×10⁻⁶, preferably greater than 1×10⁻⁴, and more preferably, greaterthan 1×10⁻² at 25° C. Likewise, for the purposes of the presentdisclosure, a strong base is defined as a base having an ionizationconstant (K_(b)) greater than 5×10⁻⁶, preferably greater than 1×10⁻⁴,and more preferably, greater than 1×10⁻² at 25° C. As used herein, theterm “acid” shall refer to a proton donor or Bronsted-Lowry acid and“base” shall refer to a hydroxide ion donor or Bronsted-Lowry base.

[0034] Strongly acidic non-aqueous electrolytes are used when the fuelcell relies on proton ions, for example proton exchange membrane fuelcells (PEMFCs), while strongly basic non-aqueous electrolytes are usedwhen the fuel cell relies on hydroxide ions, such as in alkaline fuelcells (AFCs). Thus, non-aqueous electrolytes suitable for use in thepresent disclosure are typically strong acids and bases. Alternatively,non-aqueous electrolytes, such as those described in U.S. Pat. No.5,965,054, which is hereby incorporated by reference in its entirety forall purposes, that are weak acids and bases may be modified by theaddition of electrochemically active ions, i.e. protons or hydroxideions, to form strongly acidic or strongly basic non-aqueouselectrolytes.

[0035] As discussed above, fuel processor 18 may utilize any suitablehydrogen-producing mechanism including steam reforming, autothermalreforming, electrolysis, irradiation, pyrolysis and catalytic partialoxidation. An example of a suitable fuel processor 18 is a steamreformer. An example of a suitable steam reformer is shown in FIG. 4 andindicated generally at 130. Reformer 130 includes a reforming, orhydrogen-producing, region 132 that includes a steam reforming catalyst134. Alternatively, reformer 130 may be an autothermal reformer thatincludes an autothermal reforming catalyst. In reforming region 132, areformate stream 136 is produced from the water and carbon-containingfeedstock forming feed stream 20. The reformate stream typicallycontains hydrogen gas and impurities, and therefore is delivered to aseparation region, or purification region, 138, where the hydrogen gasis purified. In separation region 138, the hydrogen-containing stream isseparated into one or more byproduct streams, which are collectivelyillustrated at 140, and a hydrogen-rich stream 142 by any suitablepressure-driven separation process. In FIG. 4, hydrogen-rich stream 142is shown forming stream 17.

[0036] An example of a suitable structure for use in separation region138 is a membrane module 144, which contains one or more hydrogenpermeable metal membranes 146. Examples of suitable membrane modulesformed from a plurality of hydrogen-selective metal membranes aredisclosed in U.S. Pat. No. 6,221,117, the complete disclosure of whichis hereby incorporated by reference in its entirety for all purposes. Inthat patent, a plurality of generally planar membranes are assembledtogether into a membrane module having flow channels through which animpure gas stream is delivered to the membranes, a purified gas streamis harvested from the membranes and a byproduct stream is removed fromthe membranes. Gaskets, such as flexible graphite gaskets, are used toachieve seals around the feed and permeate flow channels. Also disclosedin the above-identified application are tubular hydrogen-selectivemembranes, which also may be used. Other suitable membranes and membranemodules are disclosed in U.S. Pat. No. 6,547,858, which was filed onJul. 19, 2000 and is entitled “Hydrogen-Permeable Metal Membrane andMethod for Producing the Same,” the complete disclosure of which ishereby incorporated by reference in its entirety for all purposes. Othersuitable fuel processors are also disclosed in the incorporated patentapplications.

[0037] The thin, planar, hydrogen-permeable membranes are preferablycomposed of palladium alloys, most especially palladium with 35 wt % to45 wt % copper. These membranes, which also may be referred to ashydrogen-selective membranes, are typically formed from a thin foil thatis approximately 0.001 inches thick. It is within the scope of thepresent disclosure, however, that the membranes may be formed fromhydrogen-selective metals and metal alloys other than those discussedabove, hydrogen-permeable and selective ceramics, or carboncompositions. The membranes may have thicknesses that are larger orsmaller than discussed above. For example, the membrane may be madethinner, with commensurate increase in hydrogen flux. Thehydrogen-permeable membranes may be arranged in any suitableconfiguration, such as arranged in pairs around a common permeatechannel as is disclosed in the incorporated patent applications. Thehydrogen permeable membrane or membranes may take other configurationsas well, such as tubular configurations, which are disclosed in theincorporated patents.

[0038] Another example of a suitable pressure-separation process for usein separation region 138 is pressure swing absorption (PSA). In apressure swing adsorption (PSA) process, gaseous impurities are removedfrom a stream containing hydrogen gas. PSA is based on the principlethat certain gases, under the proper conditions of temperature andpressure, will be adsorbed onto an adsorbent material more strongly thanother gases. Typically, it is the impurities that are adsorbed and thusremoved from reformate stream 136. The success of using PSA for hydrogenpurification is due to the relatively strong adsorption of commonimpurity gases (such as CO, CO₂, hydrocarbons including CH₄, and N₂) onthe adsorbent material. Hydrogen adsorbs only very weakly and sohydrogen passes through the adsorbent bed while the impurities areretained on the adsorbent. Impurity gases such as NH₃, H₂S, and H₂Oadsorb very strongly on the adsorbent material and are therefore removedfrom stream 136 along with other impurities. If the adsorbent materialis going to be regenerated and these impurities are present in stream136, separation region 138 preferably includes a suitable device that isadapted to remove these impurities prior to delivery of stream 136 tothe adsorbent material because it is more difficult to desorb theseimpurities.

[0039] Adsorption of impurity gases occurs at elevated pressure. Whenthe pressure is reduced, the impurities are desorbed from the adsorbentmaterial, thus regenerating the adsorbent material. Typically, PSA is acyclic process and requires at least two beds for continuous (as opposedto batch) operation. Examples of suitable adsorbent materials that maybe used in adsorbent beds are activated carbon and zeolites, especially5 Å (5 angstrom) zeolites. The adsorbent material is commonly in theform of pellets and it is placed in a cylindrical pressure vesselutilizing a conventional packed-bed configuration. It should beunderstood, however, that other suitable adsorbent materialcompositions, forms and configurations may be used.

[0040] Reformer 130 may, but does not necessarily, further include apolishing region 148, such as shown in FIG. 5. Polishing region 148receives hydrogen-rich stream 142 from separation region 138 and furtherpurifies the stream by reducing the concentration of, or removing,selected compositions therein. For example, when stream 142 is intendedfor use in a fuel cell stack, such as stack 12, compositions that maydamage the fuel cell stack, such as carbon monoxide and carbon dioxide,may be removed from the hydrogen-rich stream. The concentration ofcarbon monoxide should be less than 10 ppm (parts per million) toprevent the control system from isolating the fuel cell stack.Preferably, the system limits the concentration of carbon monoxide toless than 5 ppm, and even more preferably, to less than 1 ppm. Theconcentration of carbon dioxide may be greater than that of carbonmonoxide. For example, concentrations of less than 25% carbon dioxidemay be acceptable. Preferably, the concentration is less than 10%, evenmore preferably, less than 1%. Especially preferred concentrations areless than 50 ppm. It should be understood that the acceptable minimumconcentrations presented herein are illustrative examples, and thatconcentrations other than those presented herein may be used and arewithin the scope of the present disclosure. For example, particularusers or manufacturers may require minimum or maximum concentrationlevels or ranges that are different than those identified herein.

[0041] Region 148 includes any suitable structure for removing orreducing the concentration of the selected compositions in stream 142.For example, when the product stream is intended for use in a PEM fuelcell stack or other device that will be damaged if the stream containsmore than determined concentrations of carbon monoxide or carbondioxide, it may be desirable to include at least one methanationcatalyst bed 150. Bed 150 converts carbon monoxide and carbon dioxideinto methane and water, both of which will not damage a PEM fuel cellstack. Polishing region 148 may also include another hydrogen-producingdevice 152, such as another reforming catalyst bed, to convert anyunreacted feedstock into hydrogen gas. In such an embodiment, it ispreferable that the second reforming catalyst bed is upstream from themethanation catalyst bed so as not to reintroduce carbon dioxide orcarbon monoxide downstream of the methanation catalyst bed.

[0042] Steam reformers typically operate at temperatures in the range of200° C. and 700° C., and at pressures in the range of 50 psi and 1000psi, although temperatures outside of this range are within the scope ofthe disclosure, such as depending upon the particular type andconfiguration of fuel processor being used. Any suitable heatingmechanism or device may be used to provide this heat, such as a heater,burner, combustion catalyst, or the like. The heating assembly may beexternal the fuel processor or may form a combustion chamber that formspart of the fuel processor. The fuel processing system, fuel cellsystem, an external source, or a combination thereof, may provide thefuel for the heating assembly.

[0043] It is within the scope of the present disclosure that any othertype of fuel processor may be used, such as those discussed above, andthat any other suitable source of hydrogen gas may be used. Examples ofother sources of hydrogen include a storage device, such as a storagetank or hydride bed, containing a stored supply of hydrogen gas.

[0044] Although discussed above in terms of a PEM fuel cell, it iswithin the scope of the present disclosure that non-aqueousBronsted-Lowry electrolyte 60 may be implemented with other forms offuel cells. For example, the system may be implemented with other lowand moderate temperature fuel cells, such as alkaline fuel cells (AFCs)or direct methanol fuel cells (DMFCs).

Industrial Applicability

[0045] The present disclosure is applicable to energy-producing systems,and more particularly to fuel cells and fuel cell systems.

[0046] It is believed that the disclosure set forth above encompassesmultiple distinct inventions with independent utility. While each ofthese inventions has been disclosed in its preferred form, the specificembodiments thereof as disclosed and illustrated herein are not to beconsidered in a limiting sense as numerous variations are possible. Thesubject matter of the disclosure includes all novel and non-obviouscombinations and subcombinations of the various elements, features,functions and/or properties disclosed herein. Similarly, where theclaims recite “a” or “a first” element or the equivalent thereof, suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.

[0047] It is believed that the following claims particularly point outcertain combinations and subcombinations that are directed to one of thedisclosed inventions and are novel and non-obvious. Inventions embodiedin other combinations and subcombinations of features, functions,elements and/or properties may be claimed through amendment of thepresent claims or presentation of new claims in this or a relatedapplication. Such amended or new claims, whether they are directed to adifferent invention or directed to the same invention, whetherdifferent, broader, narrower or equal in scope to the original claims,are also regarded as included within the subject matter of theinventions of the present disclosure.

I claim:
 1. A fuel cell, comprising: an anode region adapted to receivean anode feedstock comprising at least one composition that containschemically bound hydrogen and which liberates protons at an anode of thefuel cell; a cathode region adapted to receive a stream containingoxygen; and an electrolytic barrier separating the anode region from thecathode region, wherein the electrolytic barrier includes a non-aqueousliquid electrolyte, and further wherein the fuel cell has an operatingtemperature of less than 300° C.
 2. The fuel cell of claim 1, whereinthe non-aqueous liquid electrolyte is acidic.
 3. The fuel cell of claim2, wherein the non-aqueous liquid electrolyte has an acid ionizationconstant (K_(a)) greater than 5×10⁻⁶ at 25° C.
 4. The fuel cell of claim2, wherein the non-aqueous liquid electrolyte has an acid ionizationconstant (K_(a)) in the range of 5×10⁻⁶ and 7×10⁻³ at 25° C.
 5. The fuelcell of claim 2, wherein the non-aqueous liquid electrolyte has an acidionization constant (K_(a)) greater than 1×10⁻² at 25° C.
 6. The fuelcell of claim 1, wherein the non-aqueous liquid electrolyte is basic. 7.The fuel cell of claim 6, wherein the non-aqueous liquid electrolyte hasa base ionization constant (K_(b)) greater than 5×10⁻⁶ at 25° C.
 8. Thefuel cell of claim 6, wherein the non-aqueous liquid electrolyte has abase ionization constant (K_(b)) greater than 1×10⁻² 25° C.
 9. The fuelcell of claim 1, wherein the non-aqueous liquid electrolyte includes asolvent and a solute.
 10. The fuel cell of claim 9, wherein the soluteis an acidic solute.
 11. The fuel cell of claim 10, wherein the acidicsolute comprises at least one component selected from the groupconsisting of: hydrogen hexafluorophosphate, hydrogen tetraphenylborate,sulfuric acid, and perchloric acid.
 12. The fuel cell of claim 9,wherein the solute is a basic solute.
 13. The fuel cell of claim 12,wherein the basic solute includes a tetraalkylammonium hydroxide. 14.The fuel cell of claim 1, wherein the non-aqueous liquid electrolytecontains less than 5% water on a molar basis.
 15. The fuel cell of claim1, wherein the boiling point of the non-aqueous liquid electrolyte is atleast 90° C.
 16. The fuel cell of claim 1, wherein the boiling point ofthe non-aqueous liquid electrolyte is at least 130° C.
 17. The fuel cellof claim 1, wherein the boiling point of the non-aqueous liquidelectrolyte is at least 150° C.
 18. The fuel cell of claim 1, whereinthe melting point of the non-aqueous liquid electrolyte is less than 5°C.
 19. The fuel cell of claim 1, wherein the melting point of thenon-aqueous liquid electrolyte is less than 0° C.
 20. The fuel cell ofclaim 1, wherein the operating temperature of the fuel cell is between15° C. and 200° C.
 21. The fuel cell of claim 1, wherein the operatingtemperature of the fuel cell is between 15° C. and 150° C.
 22. The fuelcell of claim 1, wherein the operating temperature of the fuel cell isbetween 0° C. and 100° C.
 23. The fuel cell of claim 1, wherein theoperating temperature of the fuel cell is above 100° C.
 24. The fuelcell of claim 1, wherein the operating temperature of the fuel cell isbetween 150° C. and 300° C.
 25. The fuel cell of claim 1, wherein theoperating temperature of the fuel cell is above the boiling point ofwater at the operating conditions of the fuel cell.
 26. The fuel cell ofclaim 1, in combination with a source of the anode feedstock.
 27. Thefuel cell of claim 26, wherein the anode feedstock includes hydrogen gasand the source is adapted to deliver a stream containing hydrogen gas tothe anode region of the fuel cell.
 28. The fuel cell of claim 26,wherein anode feedstock includes methanol and the source is adapted todeliver a stream containing methanol to the anode region of the fuelcell.
 29. The fuel cell of claim 26, wherein the anode feedstockincludes hydrazine and the source is adapted to deliver a streamcontaining hydrazine to the anode region of the fuel cell.
 30. The fuelcell of claim 26, wherein the anode feedstock includes formaldehyde andthe source is adapted to deliver a stream containing formaldehyde to theanode region of the fuel cell.
 31. The fuel cell of claim 26, whereinthe anode feedstock includes ethanol and the source is adapted todeliver a stream containing ethanol to the anode region of the fuelcell.
 32. The fuel cell of claim 26, wherein the source of the anodefeedstock includes at least one storage tank containing the anodefeedstock.
 33. The fuel cell of claim 26, wherein the source includes adevice adapted to produce the anode feedstock through a chemicalreaction.
 34. The fuel cell of claim 26, wherein the source includes afuel processor with a hydrogen-producing region.
 35. The fuel cell ofclaim 34, wherein the fuel processor is a steam reformer that includes areforming catalyst.
 36. The fuel cell of claim 34, wherein the fuelprocessor is adapted to produce the anode feedstock by at least one ofthe following methods: partial oxidation of a carbon-containingfeedstock, electrolysis of water and irradiation of a carbon-containingfeedstock.
 37. The fuel cell of claim 34, wherein the hydrogen-producingregion of the fuel processor is adapted to receive a feed stream thatincludes a carbon-containing feedstock and to produce therefrom a mixedgas stream containing hydrogen gas and other gases.
 38. The fuel cell ofclaim 37, wherein the fuel processor further comprises a separationregion adapted to receive the mixed gas stream and to separate the mixedgas stream into an at least substantially pure hydrogen stream and abyproduct stream containing substantially the other gases.
 39. The fuelcell of claim 38, wherein the separation region of the fuel processorincludes at least one hydrogen-selective membrane.
 40. The fuel cell ofclaim 38, wherein the fuel processor includes a polishing catalyst bedadapted to increase the purity of the at least substantially purehydrogen stream.
 41. A fuel cell system, comprising: means for producingan anode feedstock containing hydrogen gas; at least one fuel cellcomprising: an anode region adapted to receive at least a portion of theanode feedstock; a cathode region adapted to receive a stream containingoxygen; and an electrolytic barrier separating the anode region from thecathode region, wherein the electrolytic barrier includes an acidic orbasic non-aqueous electrolyte; wherein the non-aqueous electrolyteincludes an organic ionic liquid and has an acid ionization constant(K_(a)) greater than 5×10⁻⁶ at 25° C. if the non-aqueous electrolyte isan acid and a base ionization constant (K_(b)) greater than 5×10⁻⁶ at25° C. if the non-aqueous electrolyte is a base; and further wherein thefuel cell has an operating temperature of less than 300° C.
 42. The fuelcell system of claim 41, wherein the non-aqueous electrolyte is an acid.43. The fuel cell system of claim 41, wherein the non-aqueouselectrolyte is a base.
 44. The fuel cell system of claim 41, wherein thenon-aqueous electrolyte further-includes sulfuric acid.
 45. The fuelcell system of claim 41, wherein the organic ionic liquid is a solventand the non-aqueous electrolyte further includes an acidic or basicsolute.
 46. The fuel cell system of claim 45, wherein the acidic soluteis selected from the group consisting of: hydrogen hexafluorophosphate,hydrogen tetraphenylborate, sulfuric acid, and perchloric acid.
 47. Thefuel cell system of claim 45, wherein the basic solute is atetraalkylammonium hydroxide.
 48. The fuel cell system of claim 41,wherein the non-aqueous electrolyte has a K_(a) greater than 1×10⁻² at25° C. if the electrolyte is an acid and a K_(b) greater than 1×10⁻² 25°C. if the electrolyte is a base.
 49. The fuel cell system of claim 41,wherein the non-aqueous electrolyte has a K_(a) greater than 1×10⁻⁴ at25° C. if the electrolyte is an acid and a K_(b) greater than 1×10⁻⁴ at25° C. if the electrolyte is a base.
 50. The fuel cell system of claim41, wherein the non-aqueous electrolyte contains less than 5% water on amolar basis.
 51. The fuel cell system of claim 41, wherein the boilingpoint of the non-aqueous electrolyte is at least 90° C.
 52. The fuelcell system of claim 41, wherein the boiling point of the non-aqueouselectrolyte is at least 130° C.
 53. The fuel cell system of claim 41,wherein the boiling point of the non-aqueous electrolyte is at least150° C.
 54. The fuel cell system of claim 41, wherein the melting pointof the non-aqueous electrolyte is less than 5° C.
 55. The fuel cellsystem of claim 41, wherein the melting point of the non-aqueouselectrolyte is less than 0° C.
 56. The fuel cell system of claim 41,wherein the operating temperature of the fuel cell is above the boilingpoint of water at the operating conditions of the fuel cell.